1. Field of the Invention
This invention is related to a CATV (cable television) system and, in particular, is directed to a system that acts as an interface between a broadcast satellite system and the CATV system.
2. Description of the Prior Art
When a broadcast satellite system and a prior art CATV system are combined with each other, the combination can be as shown in FIG. 1.
In FIG. 1, reference numeral 1 designates a broadcast satellite (BS), 2 a CATV center, 3 a terminal (terminal receiver) installed at each user's home and 6 a coaxial cable. When a radio wave of, for example, 4 channels (in Japan, a radio wave of 8 channels is permitted at maximum) is transmitted from the broadcast satellite 1, this radio wave is received by a parabolic antenna 21. The radio wave or signal received by the parabolic antenna 21 is frequency-converted to an intermediate frequency signal by an outdoor unit 22 (including a BS (broadcast satellite) converter which frequency-converts a signal of super high frequency of 12 GH.sub.z to that of 1 GH.sub.z, which is disposed behind the parabolic antenna 21. The intermediate frequency signal thus frequency-converted is supplied through a coaxial cable to four indoor units 231 to 234 in which it is FM (frequency-modulation) -demodulated. Thereafter, video signals V.sub.1 to V.sub.4 are respectively demodulated from base band signals of the respective channels and the corresponding audio signals S.sub.1 to S.sub.4 are respectively demodulated from the PSK-modulated signals (phase shift keying modulated signals). These video signals V.sub.1 to V.sub.4 and audio signals S.sub.1 to S.sub.4 are respectively supplied to modulators 241 to 244, in which they are modulatd to television signals T.sub.1 to T.sub.4 as shown in FIG. 2 the (in FIG. 2, respective channels corresponding to television signals T1-T4 are shown adjoining to one another for simplicity). Thus, the video and audio signals are modulated to conventional television channel frequencies. The television signals T.sub.1 to T.sub.4 are delivered through a mixing circuit 25 to the coaxial cable 6. Accordingly, at the terminal 3, it is possible to view a television program of an arbitrary channel of the broadcast satellite.
Furthermore, in this kind of broadcast satellite system, the audio signals S.sub.1 to S.sub.4 are modulated in the form of PCM )pulse code modulated) digital signals and PSK-modulated on an audio sub-carrier of 5.727272 MHz and then respectively transmitted from the broadcast satellite 1. And in the indoor units 231 to 234, the PCM signals are respectively decoded to the audio signals S.sub.1 to S.sub.4. In other words, up to the indoor units 231 to 234, each of the audio signals S.sub.1 to S.sub.4 is of extremely superior quality whose band width is 20 kHz and whose dynamic range is wide.
However, at the stage when the audio signals S.sub.1 to S.sub.4 are delivered to the coaxial cable 6, the audio signals S.sub.1 to S.sub.4 are respectively modulated to the same FM signals as those used in the ordinary television broadcasting. Also, as shown in FIG. 2, the audio signals S.sub.1 to S.sub.4 are multiplexed with the video signals V.sub.1 to V.sub.4 in frequency-divided manner so that the tone quality presented by the PCM digital signal is deteriorated relative to that which is available from satellite 1.